In conventional facilities producing chemical pulp utilizing continuous digesters, the typical manner of handling the extracted black liquor from the continuous digester is to pass the black liquor to a first flash tank, and then to a second flash tank. The concentrated black liquor from the second flash tank is then passed to evaporators to be concentrated, steam being supplied to the first effect of the multiple effect evaporators, to provide the heat for effecting evaporation of the black liquor so that it is concentrated. The concentrated black liquor is then burned and/or subjected to chemical recovery, following known procedures. A conventional number 2 flash tank is a bulky, expensive, and space-consuming piece of equipment which heretofore has been found to be necessary, although undesirable.
In the practice of conventional continuous chemical pulp production procedures, oftentimes there is also the production of an excess amount (volume) of liquor entering the digester than required for cooking. The high pressure feeder, which feeds the comminuted cellulosic fibrous material (e.g. wood chips) slurry ultimately to the continuous digester (either directly or through an impregnation vessel, etc.) has a recirculation system which is designed to supply the desired amount of liquid to the chips so that proper feeding is effected. However oftentimes there is an excess volume of make-up liquor since the chips inherently have moisture in them, white liquor is added, and there is condensate from chips bin steaming, or from the steaming vessel steaming. Typically the excess make-up liquor is added to a white liquor supply to an impregnation vessel or the digester, utilizing a level tank. This procedure is less than ideal, however, since it utilizes a level tank as a distinct piece of equipment.
According to the present invention an apparatus and procedures are provided which overcome some of the undesirable aspects associated with the production of chemical pulp from conventional continuous digesters.
According to one aspect of the present invention, the number 2 flash tank is eliminated. The heated concentrated black liquor from the number 1 flash tank, instead of being passed to the number 2 flash tank, is used as the heating fluid for a multiple effect evaporator. This results in effective recovery of the energy of the black liquor, while it is being concentrated, does not normally require any additional effects for the evaporator system than would otherwise be provided, while allowing elimination of the undesirable number 2 flash tank.
The heated concentrated black liquor from the number 1 flash tank is passed into heat exchange relationship with other portions of the extracted black liquor which have had more of their heat content removed, resulting in evaporation of the black liquor and ultimately the desired concentration thereof. The concentrated black liquor, and the vapor from the black liquor evaporation, are handled in exactly the same manner as in the prior art (i.e. by any number of conventional techniques).
According to another aspect of the present invention, in addition to effecting concentration of itself, the heated black liquor is utilized to evaporate the excess make-up liquor, the evaporated excess make-up liquor then being utilized as steam for the chips bin and/or black liquor pre-evaporators, etc. Excess make-up liquor withdrawn from the conventional in-line drainers is passed directly to an evaporator as the evaporating liquid for the evaporator, so that the level tank--previously considered necessary--is eliminated. In subsequent effects of a multiple effect evaporator, black liquor is also evaporated. That portion of the excess make-up liquor that is not evaporated in the evaporator is merely returned to the high pressure inlet line to the high pressure feeder.
Where additional heat content is necessary to effect the desired amount of evaporation, the additional heat can be supplied from the steam relief of the conventional steaming vessel and excess steam from number one flash tank, the steam from the steam relief and number one flash tank being fed to the first effect of the multiple effect evaporator, or make up liquor evaporator, along with the heated black liquor.
By practicing the present invention, not only are the heretofore considered necessary number 2 flash tank and make-up liquor level tank eliminated, other advantages accrue. For instance pulp production can be achieved using somewhat less energy since some of the water is eliminated from the digester (through evaporation of the excess make-up liquor), and therefore the need to heat the material entering or which has entered the digester with high pressure steam is minimized. Additionally, there is thus less extraction that is necessary throughout the continuous digestion process, or the same amount of extraction can be practiced by achieving better washing. This would allow minimizing the surface area of extraction screens, and/or effecting better washing. Also, the top circulation temperature can be stabilized, with resulting possibility increasing the pulp quality. Also, some portion of the fresh low pressure steam that is fed to the chips bin or steaming vessel can be saved.
It is the primary object of the present invention to provide effective yet efficient evaporation of black liquor and/or make-up liquor. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.